Today cancer is the second most important disease leading to death in the developing and developed countries. Cancer claims over six million lives globally each year. Although the efficacy of chemotherapy and other standard therapies for the majority of cancer types has been improved during the last decades, the treatment of most human malignances is still facing high mortality rates. Moreover, toxic side effects of the current chemotherapeutical drugs are often causing a severe reduction in the quality of life. Therefore, the development of novel potent, but non-toxic anti-cancer agents is worth a continuous effort. It was been discovered, that several plant derived natural products may serve as effective anticancer drugs, among them are plant triterpenes, for example betulin and betulinic acid (Setzer, W. N. and Setzer, M. C., Mini-Rev. Med. Chem., 3:540-556, 2003 and Kinghorn, A. D. et al., Planta Med., 70:691-705, 2004). Betulin is present in large quantities in the outer birch bark of numerous species of birch trees. For example the outer bark of Betula paparifera contains nearly 5-18% betulin (U.S. patent Ser. No. 09/371,298). Betulinic acid can also derive from several natural sources (U.S. Pat. Nos. 6,264,998, 6,175,035 and 6,943,260 and WO 03/066659) or it can be chemically derived from betulin. Pezzuto, J. M. et al., (U.S. Pat. No. 5,804,575) disclosed two processes for the synthesis of betulinic acid from betulin. Krasutsky, P. A. et al., (U.S. Pat. No. 6,232,481) disclosed a further multi-step process for the synthesis of this triterpenoid. Finally, Menard, H. et al., (WO 2006/063464) also reported the synthesis of betulinic acid from betulin with an electrochemical oxidation reaction. Both betulin and betulinic acid were reported to display several biological effects including anti-inflammatory, antiviral, antimalarial, anticancer, antiseptic, antimicrobial and antifeedant activities (Dzubak, P. et al., Nat. Prod. Rep., 23:294-311, 2006 and Tolstikova, T. G. et al., Russ. J. Bioorg. Chem., 32:37-49, 2006). However amongst all the aforementioned activities, betulinic acid has been found to exhibit particular anticancer and anti-HIV activities (WO 98/51294, U.S. Pat. No. 5,869,535, US 2006/0159783, WO 96/39033 and U.S. Pat. No. 5,679,828). Betulinic acid was originally considered a melanoma-specific cytotoxic agent. Pisha, E. et al., (Nat. Med., 1:1046-1051, 1995) reported that betulinic acid has an unexpected selective antitumor activity against human melanoma cells, MEL-1, MEL-2, and MEL-4 and confirmed this effectiveness using athymic mice. DasGupta, T. K. et al. and Pezzuto, J. M et al., (WO 96/29068 and U.S. Pat. No. 5,962,527) also disclosed that betulinic acid and its derivatives are useful on treatment of human melanoma. However recent evidence indicates that betulinic acid possesses a broader spectrum of cytotoxic activity against other cancer cell lines. Ramadoss, S. et al. (U.S. Pat. No. 6,048,847) reported for the first time the anti-leukemia, anti-lymphoma, anti-prostate cancer and anti-lung cancer activity of betulinic acid and its derivatives. Debatin, K. M. et al. (U.S. Pat. No. 6,369,109) reported the activity of betulinic acid and derivatives against neuroblastoma cells and Mukherjee, R. et al. (US 2006/0159783) reported the anticancer activity of betulinic acid against cancers of colon, intestine, stomach, breast, lung, cervix, ovary, prostate, oral cavity, larynx, liver, pancreas, kidney, bladder, endothelial cells, glioblastoma, leukemia and myeloma, using a herbal extract rich in this triterpenoid. The molecular mechanism of betulinic acid effects on cancer cells is still subject of continuous investigations. However, this compound seems to induce apoptosis via the activation of caspases, independent of cellular p53 genes status and CD95 activation, (Fulda, S. et al., Cancer Res., 57:4956-4964, 1997 and Wick, W. et al., J. Pharmacol. Exp. Ther., 289:1306-1312, 1999), by a direct effect on mitochondria (Fulda, S. et al., J. Biol. Chem., 273:33942-33948, 1998). The apoptosis inducing ability, the apparent lack of toxicity on normal cells (Zuco, V. et al., Cancer Lett., 175:17-25, 2002), and the favorable therapeutic index, have made betulinic acid an attractive and a very promising anticancer agent (Eiznhamer, D. A. and Xu, Z.-Q., IDrugs, 7:359-373, 2004). In the past few years, there has been a great deal of interest in the synthesis and evaluation of new derivatives of betulin and betulinic acid for their biological activities. The structure of betulin and betulinic acid is based on a 30-carbon skeleton which has three sites available for simple chemical modifications C-3, C-20 and C-28. Modifications of the parent structure of these compounds at these positions can produce potentially important derivatives, more effective compared with the starting ones, which may be developed as antitumor drugs. For instance, the use of betulin, betulinic acid and derivatives thereof for cancer chemoprevention and chemotherapy is described in US 2002/0652352, US 2006/0194774, WO 2007/101873, WO 2007/112043, WO 2008/063318, US 2008/0293682, US 2008/0167254 among others. Although, all the above mentioned reports collectively disclose a large number of betulin and betulinic acid derivatives, a need still exists for new derivatives, which are not only potent, but also clinically safe and moreover, have better pharmacokinetic properties. Thus, an object of this invention is the synthesis of new betulin and betulinic acid derivatives that specifically treat, prevent, inhibit, regulate and/or modulate cancer. Nitrogen-containing derivatives of betulin and betulinic acid, such as amine derivatives, oxime derivatives, amino acid conjugates, amide derivatives, hydrazine and hydrazone derivatives, imidazolyl derivatives, and other N-heterocyclic derivatives have been reported to possess antiproliferative effect against tumor cell lines (U.S. Pat. No. 5,869,335, WO 2007/101873, WO 2007/112043 and US 2008/0293682). In our efforts to find molecules which are not only potent therapeutically but also acceptable clinically, we have found that the introduction of an imidazole, methylimidazole or triazole ring at C-3 and C-28 positions of betulin, betulinic acid and several derivatives, provides the desired characteristics which forms the basis of the present invention.